Mass-unbalance in rotating machines acts as an undesirable synchronous disturbance at all operating frequencies. With the use of magnetic bearings as active control elements it is possible to adaptively compensate for mass-unbalance loading, thus eliminating vibration and fatigue in the support structure of the rotor.
There are two former methods of performing mass unbalance compensation. In conventional autobalancing a notch filter centered at the frequency of rotation is inserted in the control loop. Because of the notch filter, the control law has no gain at the frequency of rotation; hence, the bearings become "soft" at this frequency. No bearing force is generated, and the rotor pivots about its center of mass as if it were in free space. The second former method is a model-based balancing approach. Here an estimate of the mass-unbalance eccentricity is obtained by subtracting the output of a plant model or observer from the measured rotor position.
In conventional autobalancing the notch frequency may be within or near the bandwidth of the control system (if the control system has a very low bandwidth balancing would be unnecessary). The disadvantage is that the stability margins of the system are severely degraded due to the phase lag contributed by the notch filter. Disturbance rejection properties near the notch frequency are also degraded. Furthermore, when the rotor spins through its critical speeds (i.e., bending modes) the notch filter must be disabled to avoid instability. This form of autobalancing is inappropriate for moving platform applications (e.g., jet engines) because such applications require high-bandwidth controllers and guaranteed stability margins.
Model-based balancing requires the implementation of an observer and may be sensitive to modeling errors and time-varying plant parameters. The disadvantage is that the balancing achieved can be only as good as the accuracy of the model used. Also, model-based balancing tends to restrict system bandwidth.
Magnetic bearing suspension systems are but one environment in which unwanted dynamic vibrations occur. They can occur in any type of rotary system with any type of suspension apparatus. Moreover, dynamic vibrations, rotary or not, can occur in any kind of mechanical or acoustic environment. For example: environmental vibrations in a duct caused by a fan; unwanted vibrations in a vehicle seat caused by the vehicle engine.